Variable optical attenuators potentially have a variety of important uses in optical communication systems. For instance, in wavelength division multiplexed (WDM) systems they could be used to tune the wavelength dependent gain of fiber amplifiers such that all wavelengths of interest have substantially the same gain. Variable optical attenuators exemplarily could also be used to compensate for variable input strengths to achieve a constant output, or to compensate for variable path length attenuation to produce equal strength signals. Desirably, variable attenuators for optical communication systems have attenuation up to about 20-25 dB, and insertion loss of about 1.5 dB or less.
The prior art knows several types of variable optical attenuators, including mechanical ones. See, for instance, S. Masuda, Applied Optics, Vol. 19, p. 2435 (1980), and W. L. Emkey, Optics Letters, Vol. 8, p. 94 (1983). Mechanical devices are unlikely to achieve the speed required for high bit rate systems. R. Wolfe et al., Applied Physics Letters, Vol. 58, p. 1733 (1991) suggested a device based on a single domain wall in a magnetooptic waveguide. Such a device however would be costly to fabricate, and have high insertion loss for coupling to fiber. Fukushima et al., (Optical Society of America TOPS on Optical Amplifiers and Their Applications, 1996, Vol. 5, 1996 OAA Program Committee (eds), pp. 249-252), have disclosed a thick magnetic film with in-plane magnetization, where the direction of the magnetization in the plane depends on a relatively small magnetic bias field. The Faraday rotation in the direction of light propagation depends on the cosine of the angle between the magnetization and the direction of propagation. This approach has drawbacks with the in-plane coupling of light and the non-uniformity of magnetization with bias field because of the cubic magnetic anisotropy of the film.
In view of the many potential uses of variable optical attenuators, it would be desirable to have available a compact, low power device having a significant tuning range. This application discloses such a device.
It is known that rare earth iron garnets (RIGs) are ferromagnetic, with three inequivalent metal ion sites (octahedral, tetrahedral, and dodecahedral). The net saturation magnetization M.sub.s of a RIG of interest herein is given by EQU M.sub.s (T)=.vertline..+-.M.sub.c (T)-M.sub.a (T)+M.sub.d (T).vertline.,(1)
where M.sub.c, M.sub.a and M.sub.d are the sublattice magnetizations of the dodecahedral, octahedral and tetrahedral sublattices, respectively, and T is the absolute temperature. FIG. 1 schematically shows the sublattice magnetization as a function of temperature of the three sublattices of an exemplary RIG, and FIG. 2 shows the resulting net magnetization as a function of temperature.
As can be seen from FIG. 1, the rare earth contribution, for heavy rare earth's (Gd to Yb), is large at low temperatures but is substantially negligible at high temperatures. Consequently, such RIGs can exhibit magnetic compensation (i.e., zero saturation magnetization) at some temperature below the Curie temperature. This is illustrated in FIG. 2, where the compensation temperature is about 250 K and the Curie temperature is about 500 K. The temperature dependence of the rare earth moment is highest for Gd and decreases steadily through the heavy rare earths. Data for Tb, Ho and Yb are shown in FIG. 3.
Like the saturation magnetization, the Faraday rotation .theta..sub.F of the iron garnets is also a linear combination of the sublattice magnetizations, but with different constants of proportionality. Specifically, the Faraday rotation is EQU .theta..sub.F (T,.lambda.)=C(.lambda.)M.sub.c (T)+A(.lambda.)M.sub.a (T)+D(.lambda.)M.sub.d (T), (2)
where .lambda. is the wavelength of the light that experiences the Faraday rotation and C(.lambda.), A(.lambda.) and D(.lambda.) are the wavelength dependent magnetooptic coefficients of the dodecahedral, octahedral and tetrahedral sublattices, respectively. As a result of the different constants of proportionality, the Faraday rotation of a RIG of interest herein typically does not go to zero at the compensation temperature. However, when the material passes through the magnetic compensation temperature, the sign of the saturation magnetization changes with respect to the sublattice magnetizations, and therefore with respect to the Faraday rotation.
If the material is in a saturating applied magnetic field, as occurs in many device applications, passing through the compensation temperature results in sign changes in the sublattice magnetizations, so that the net magnetization will remain aligned with the applied field. When this occurs, the Faraday rotation also changes sign (but not magnitude) in a step function, along with the changes in sublattice magnetization in accordance with equation (2) above. This is shown schematically in FIGS. 4 and 5, and graphically in FIG. 6. If such changes occur in a magnetooptic device such as an isolator, the device will also reverse its function, isolating in the forward direction and propagating light in the backward direction, typically rendering the device useless for normal operations.
It should however be noted that if the material is sufficiently coercive to remain saturated without a magnetic field present (see, for instance, U.S. Pat. No. 5,608,570), passing through the compensation temperature has the opposite effect in the absence of an applied field. Namely, the saturation magnetization changes direction as it passes through zero, and the sign of the Faraday rotation is unchanged because the sublattice magnetization will remain in the same direction.
The compensation temperature of a particular RIG material generally is determined by the combined effects of the dodecahedral ions (particular the concentration of heavy rare earths) and the diamagnetic substitution on the iron sites. As the temperature is raised or lowered through the compensation temperature in an applied magnetic field, the sublattice magnetization will change sign, as shown schematically in FIGS. 4 and 5. This typically occurs by a process of nucleation and growth of a region (or regions) of reverse sublattice magnetization into the existing domain. During this process, these magnetic domains with differently ordered sublattices are separated by a special kind of magnetic domain wall, generally referred to as a "compensation wall". The sublattices on one side of the compensation wall are oriented antiparallel to the corresponding sublattices on the other side of the compensation wall. The compensation wall has somewhat less energy than a randomly oriented domain wall, but its nucleation still requires some energy. Consequently, there is hysteresis in switching a material of uniform composition, similar to the nucleation-induced coercivity in Latching.TM. Faraday rotators.
For further detail on compensation walls, see for instance "Magnetic Garnets", Gerhard Winkler, Vieweg Tracts in Pure and Applied Physics, Vol. 5, Braunnschweig 1981, especially pages 358 to 364, and p. 672. See also U.S. Pat. No. 4,981,341.